Collecting stamps as a hobby___________ increasingly popular during the past fifty years.

A．becomes

B．became

C．has become

D．had become

有下列二叉树，对此二叉树前序遍历的结果为( )。

A．ACFXDBEYZ

B．ABEFXYZCD

C．ABCDEFXYZ

D．ABDYECFXZ